Adhesive-metal failure

Two other specific problems complicate the process of sealing pipelines with adhesives. One is that gas in gas mains is transported under 70-75 atmospheres pressure, and noticeable increase of gas main diameter is observed under this pressure. Thus, on the adhesive-metal interphase boimdary heavy internal stresses occur, which can lead to strength reduction or to failure of the adhesive joint. Joint failiu-e will also be promoted by vibration, which occurs with passage of fluid or gas through a deformed section of the pipe. 

Various modes of failure may be observed depending on the adhesive, the materials bonded and their surface pre-treatments (see Stress distribution mode of faiiure). For metal-metal peeling, very effective surface treatment is necessary to achieve cohesive failure in the adhesive otherwise, failure occurs at, or close to, one substrate surface. 

Two examples of a weak boundary layer effect are polyethylene and metal oxides. Conventional grades of polyethylene have weak, low-molecular-weight constituents evenly distributed throughout the polymer. These weak elements are present at the interface and contribute to low failing stress when polyethylene is used as an adhesive or adherend. Certain metal oxides are weakly attached to their base metals. Failure of adhesive joints made with these adherends will occur cohesively within the weak oxide layer. Weak boundary layers can be removed or strengthened by various surface treatments. 

Testing of the samples may be performed in any tensile testing apparatus. An inch at each end of the sample should be placed in the grips of the machine. Loads are applied at 1200-1400 lb per square inch of the overlap area per minute until failure. A corresponding crosshead speed of 0.05 inch/min may also be used to load the sample. The average shear stress in Ib/inch, the type of failure (i.e., cohesive or adhesive), and characteristics of the sample (i.e., adhesive, metal, sample dimensions and bonding conditions) should be recorded. The majority of stress analyses discussed in Section III use the single-lap specimen. 

The first is an adhesive-metal bonding failure resulting in elean metal showing either in parts or over the whole of the bonded area, whieh is not related to the rubber. 

CM - Cement/Metal failure failure at the primer to metal interface. For one coat adhesives, this is failure at the adhesive to metal interface. 

Finally, reference should be made to the extensive and recent durability evaluations of rubber-to-steel bonded joints in seawater. These kinds of bonds have been of especial interest to the fabricators of the deep ocean oil rigs. Stevenson ) has shown that the rate of adhesive bond failure between two metals of different electrochemical potential have special qualities that need to be understood by the designer. First, however, he demonstrated that mechanical strain in the elastomer layer did not seem to have any effect on bond durability. Furthermore, if the two adherends were electrochemically inert then the bonds were completely stable after periods as long as three years in saltwater. There was, however, an expected result when one adherend was more noble. While that adherend would be less corroded because of the electrochemical protection by the more anodic adherend, there was also a distinct increase in the rate of adhesive bond failure at that interface. 

The interfacial metal failure surface from an adhesively bonded aluminum test piece. The substrate has been grit biasted with 50 p.m alumina grit prior to bond fabrication... 

XPS images recorded from the interfacial failure surfaces of an adhesively bonded aluminum joint, prepared using an organosilane primer, with an instrument of the type shown inO Fig. 9.14. The optical mirror images are complementary views of the fracture surfaces and are mirrored along the dotted line. Visually the failure appears to be interfacial with the fracture path moving from one interface to the other at the boundary between metal and adhesive interfacial failure surfaces... 

Surface preparation, always important in obtaining optimal coatings performance, is critical for marine coatings (see Metal surface treatments). Surface preparation usually comprises about half of the total coating costs, and if inadequate may be responsible for early coating failure. Proper surface preparation includes cleaning to remove contaminants and roughening the surface to faciUtate adhesion. 

Adhesion. Commercially available 1- or 2-coat adhesive systems produce mbber failure in bonds between ethylene—acryflc elastomer and metal (14). Adhesion to nylon, polyester, or aramid fiber cord or fabric is greatest when the cord or fabric have been treated with carboxylated nitrile mbber latex. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Visually, the failed specimens appeared as shown in Fig. 34. Failure was mostly cohesive within the adhesive. However, there was a small region of apparent interfacial failure at one end of each substrate that was referred to as the metal... 

Another important application area for PSAs in the electronic industry focuses on the manufacturing, transport and assembly of electronic components into larger devices, such as computer disk drives. Due to the sensitivity of these components, contamination with adhesive residue, its outgassing products, or residue transferred from any liners used, needs to be avoided. Cleanliness of the whole tape construction becomes very critical, because residuals like metal ions, surfactants, halogens, silicones, and the like can cause product failures of the electronic component or product. Due to their inherent tackiness, acrylic PSAs are very attractive for this type of application. Other PSAs can be used as well, but particular attention has to be given to the choice of tackifier or other additives needed in the PSA formulation. The choice of release liner also becomes very critical because of the concern about silicone transfer to the adhesive, which may eventually contaminate the electronic part. 

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